Light-emitting diode (LED) has been widely used in industry and our daily lives due to its features such as a small volume and high efficiency.
Referring to FIG. 1, which shows a common LED light-emitting device, the LED light-emitting device includes a light-emitting circuit, a controllable switch Q1, a resistor Rsen, a silicon controlled rectifier U1 and a first current-limiting resistor R1. The light-emitting circuit includes a plurality of LEDs D1, . . . , Dn connected in series, wherein n is an integer greater than 1. One end of the silicon controlled rectifier U1 is grounded, and the other end thereof is connected, via the first current-limiting resistor R1, to a driving terminal Vdrive for providing a driving voltage. A source electrode of the controllable switch Q1 is connected to the light-emitting circuit, a drain electrode thereof is grounded via the resistor Rsen, and a control end (a gate electrode) thereof is connected to a first node X between the silicon controlled rectifier U1 and the first current-limiting resistor R1.
However, when there is a short circuit for LEDs in the LED light-emitting device, the driving circuit is not protected in a sufficient manner. The following description is provided in conjunction with an operation procedure of the LED light-emitting device.
As shown in FIG. 1, the driving voltage Vdrive provides an on-state voltage so as to turn on Q1, thereby to allow a current to flow through D1-Dn and Rsen. When the current flows through Rsen, a voltage drop occurs, and when a voltage across Rsen is greater than a threshold (e.g., 2.5V), the silicon controlled rectifier U1 is turned on so as to pull down a gate voltage of Q1, thereby to turn off Q1, i.e., to cut off the current flowing through the LED. After Q1 is turned off, the voltage across Rsen is dropped down to 0, and at this time the silicon controlled rectifier U1 is turned off again, so that Q1 is turned on again due to the driving voltage Vdrive. The above procedure is repeated, so as to maintain the voltage across Rsen at 2.5V.
If a voltage drop for the LED during the normal operation is V, a voltage drop for Q1 under normal operation is Vcc−nV−2.5. However, when m LEDs among D1-Dn are short-circuited (m is a positive integer less than or equal to n), the voltage drop for Q1 is changed to Vcc−(n−m)V−2.5, i.e., mV greater than the voltage drop for the LED when no short circuit occurs. As a result, the power consumption of Q1 will increase or Q1 will be damaged due to its insufficient voltage-withstanding performance, and thereby a service life of the entire circuit will be adversely affected.
The above description is given by taking the LED light-emitting circuit as an example. It should be appreciated that, the same problem also occurs for similar circuits consisting of any other light-emitting elements, which is not repeated herein.